Adagio for Strings is a piece of classical music for string orchestra by Samuel Barber.

It is Barber's own arrangement of the second movement of his String Quartet No. 1 , Op. 11, and is Barber's most popular piece, being used in the films Platoon, The Elephant Man, Amélie, Lorenzo's Oil (movie) and S1m0ne and recently being given an electronic treatment by William Orbit (a remix by Ferry Corsten sold well in both the US and the UK) and DJ Tiesto. It is also referred to in a key scene in Alice Sebold's bestselling 2002 novel The Lovely Bones. The music was used repeatedly in Platoon to add to the films powerful and emotional intensity.

The piece uses an arch form, employing and then inverting, expanding, and varying a stepwise descending melody. It has been played at the funerals of Franklin Delano Roosevelt, Princess Grace and Rainier III, Prince of Monaco.

It has also been modified into an eight-part choral work called Agnus Dei ('Lamb of God'), a favorite of choirs the world over; it has been heard in such disparate places as the computer game Homeworld, the 10th season premiere of the television show ER (TV series) (rumored), and the funeral of John F. Kennedy.

In 2004, Barber's masterpiece was voted the "saddest classical" work ever by listeners of the BBC's Today Programme, ahead of Henry Purcell's Dido's Lament and Gustav Mahler's 5th symphony.